This invention relates to printing apparatus and methods.
The invention is more particularly related to screen printing apparatus and methods.
In the manufacture of spark plugs, it is common practice to print a legend on the ceramic insulator body giving details, such as, the name of the manufacturer, a trade mark, size information and so on. Because the insulator is usually of a white or pink colour, any dark coloured printing on the insulator will be readily visible.
Various problems are met in printing on these insulator bodies. For example, the curved surface of the insulator complicates the printing process and makes it more difficult to print to a high quality. Because the print must be capable of withstanding the high temperatures met during subsequent manufacturing stages, and during use, the ink is usually ceramic-based making it highly abrasive and thereby leading to a high wear on those parts of the printing apparatus coming into contact with the ink.
Usually, the insulator is printed after glazing of the insulator, the printed insulator being subsequently fired to bake on the ink. This requires two firing operations: one to bake the glaze, and the other to bake the printing, thereby leading to increased production time. If the insulator could be glazed after printing, only one firing process would be needed during which both the ink and glaze would be fired at the same time. The difficulty with this is that the insulator is preferably arranged vertically while the glaze is sprayed on, so as to ensure even glazing. The vertical arrangement of the insulator does not, however, lend itself readily to the printing operation, especially with screen printing since the ink will flow off the printing head.
Another difficulty with previous printing processes is that, because the ink takes an appreciable time to dry, the overall production time is lengthened. This also makes it difficult to print in several colours since time must be left between the printing of each colour for the ink to dry.